Copolymers of vinyl aromatic monomers and .alpha.,.beta.-unsaturated dicarboxylic acid anhydrides have long been known in the art as useful, relatively low cost engineering thermoplastics having superior heat resistance as compared to styrene homopolymers. Styrene/maleic anhydride copolymer resins are available commercially, for example, from ARCO Chemical Company under the tradename "Dylark" and are widely used for the production of molded articles such as automobile instrument panels and other interior trim parts, appliance housings, furniture, and the like. Such copolymers are now also gaining acceptance in packaging applications such as microwave food containers.
Due to the hydrolytic instability of the .alpha.,.beta.-unsaturated dicarboxylic acid anhydride monomer component, however, such copolymers must generally be produced using bulk or non-aqueous solution polymerization processes. These processes require specialized equipment and are relatively costly due to the need to separate the final copolymer product, which typically has a high viscosity in solution or in the melt phase, from the unreacted monomer or organic solvent. Thus, if a manufacturer wishes to expand production capacity for such resins, new bulk or solution polymerization reactors and devolatilization facilities must be constructed. It would be highly desirable if existing suspension polymerization facilities, such as those commonly used to prepare styrene homopolymer beads, could be employed for the production of anhydride-containing copolymers, but this is not feasible owing to the hydrolysis which readily occurs when the anhydride monomer is exposed to water.
An alternative approach, in principle, would be to extend the anhydride-containing copolymer by blending it with another polymeric resin. However, it is well known in the polymer art that most polymers are immiscible with one another. Typically, when an attempt is made to prepare a uniform or homogeneous polyblend, the resulting mixture exhibits two or more glass transition temperatures indicating two or more separate phases. This inhomogeneity may adversely affect the physical and mechanical properties of the resin mixture and typically results in an undesirable opaque or hazy appearance in contrast to the transparency of the unblended anhydride-containing copolymer. In the art, the incompatibility of different polymers is the accepted rule and the discovery of fully miscible polymeric blends is the exception to the rule.